1. Field of the Invention
The present invention relates to broadband communication systems that use Data Over Cable Service Interface Specification (DOCSIS) media access protocol or any of its derivatives. Particularly, the present invention applies to two way satellite services that use the upstream channel descriptor of DOCSIS specification to characterize the upstream channel.
2. Background Art
Functional cable systems require high-speed packet based communication systems that are capable of supporting a wide variety of services. Services under consideration include packet telephony service, video conferencing service, T1/frame relay equivalent service and many others. In support of facilitating these services, a series of DOCSIS interface specifications have been developed. The DOCSIS specifications facilitate early definition, design, development and deployment of data-over-cable systems on a uniform, consistent, open, non-proprietary, multi-vendor interoperable basis. The intended service allows transparent bi-directional transfer Internet Protocol (IP) traffic through the data-over-cable system.
Traditional DOCSIS based satellite broadband communication systems, that allow deploying high-speed packet based communication, lack adequate specifications that allow their definition and design in a uniform manner.
In DOCSIS based broadband data communication architectures, data is transferred between a central location and many remote subscribers. The centrally located equipment can include, for example, a Headend (cable system), wireless access termination system (WATS—broadband terrestrial fixed wireless), or a satellite gateway (SMTS—satellite modem termination system).
Subscriber equipment typically includes a cable modem (CM—cable system), wireless modem (WM—broadband terrestrial fixed wireless) or a satellite modem (SM—two-way satellite).
The communication path from the gateway to the satellite modem is known as the downstream channel. Conversely, the communication path from the satellite modem to the gateway is known as the upstream channel. In standard DOCSIS based systems, traffic for the satellite modem is typically placed in time domain multiplex (TDM) fashion on the downstream channel. The satellite modem also shares upstream channels in the time domain multiplex (TDM) fashion. If the satellite modem is using a particular downstream channel, then its upstream channel traffic is assigned to one of typically several upstream channels that are associated with each downstream channel. The combination of a downstream channel and its corresponding upstream channels forms the media access control (MAC) domain of the satellite system.
In standard DOCSIS based systems, the satellite modem sends information to the gateway on the upstream channel at burst times. The burst's timing is controlled by the gateway. The information transmitted can include either data or MAC management information or both. When the Satellite modem is sending data, the transmitted data packets, or protocol data units (PDUs), are encapsulated into DOCSIS frames for the purpose of transmission. The encapsulation includes prepending a MAC header to the PDU.
In order to allow different satellite modems to share upstream channels the gateway sends bandwidth grant allocation messages, also known as media access protocol messages (MAP) to the satellite modems for each upstream in the MAC domain of the downstream. The MAP messages identify burst types and set times for each satellite modem that is granted bandwidth on a given upstream channel. The MAP messages also identify time intervals for initial ranging for satellite modems that wish to attempt to register for the first time on a channel.
The format of a DOCSIS header, associated with the MAC domain, is known from DOCSIS protocol standards. The header consists of minimum 6 bytes and includes a variable length extended header (EHDR). An EDHR field is commonly used to make “piggyback” bandwidth requests. Per DOCSIS convention, the bandwidth requests are always made in units of “mini-slots.” A mini-slot is a fixed amount of time known by both Satellite modem and gateway. The length of a mini-slot is dependent upon upstream channel parameters and is constrained to always be an integer power-of-2 factor of the upstream channel period.
Also in DOCSIS based systems, a concatenated coding scheme is used in the upstream. The concatenated code includes a DOCSIS based Reed Solomon outer code together with an inner convolutional or Turbocode. The sizes of mini-slots are typically configured such that a mini-slot contains a fairly small integer number of bytes of the Reed-Solomon outer code data (e.g., 8 bytes).
When using the piggyback extended header, the EHDR field can be used to request a maximum of 255 additional mini-slots per upstream PDU. The 255 mini-slot limitation cannot be enlarged in standard DOCSIS based applications because the piggyback request field is limited to one byte (per DOCSIS standard). For example, if each mini-slot contains 8 bytes of outer code information, the satellite modem could make a request for a maximum of 8×255=2040 bytes. The largest Ethernet frame is 1518 bytes. With reasonable definitions for Reed-Solomon codeword size, 2040 bytes of outer code data is adequate to be requested within the largest Ethernet frame.
The long delays involved in satellite applications make it highly desirable to use the piggyback method of requesting bandwidth rather than the contention based request mechanism. The limitation of the standard DOCSIS piggyback method becomes apparent when a large amount of upstream data is to be transmitted by the Satellite modem. In this case, if the Satellite modems were limited by the standard DOCSIS piggyback approach, a maximum of 255 mini-slots could be requested with each upstream PDU transmitted.
If the satellite modem is transmitting 1518 byte PDUs, it must wait for the packet to propagate from the Satellite modem to the gateway (approximately 0.25 seconds for geostationary satellites) and for the next grant to arrive from gateway (another 0.25 seconds). Since the round trip time is 0.5 seconds, a maximum of approximately 2 packets per second could be transferred, yielding a maximum data rate of about 3 kbytes per second. This data rate is far too small to be useful in current broadband satellite applications.
What is needed, therefore, is a mechanism to allow the satellite modem to request additional bandwidth from the gateway in order to facilitate higher communication data rates. What is also needed is definition, design, and development of data-over-cable satellite systems on a uniform and consistent basis.